Early efforts to burn solid hydrocarbons in the ramjet mode encountered difficulties in reliable flame holding. Ramjet technology, using oxidizer-free solid fules, developed rapidly after the role of the sudden expansion flame holder as a reliable method of flame stabilization for SFRJs was recognized.
At first, it might appear that the application of the sudden expansion flame holder to SFRJs represents no new technology since the technique has been used successfully for many years in liquid ramjet combustors.
However, a major difference exists between the two systems. In the liquid ramjet, fuel and air are premixed to the desired equivalence ratio. This assures combustion at proper flow speed by dumping into a sudden expansion chamber. In the SFRJ, air alone is dumped into the sudden expansion region.
In the SFRJ, a recirculation zone is formed in the expansion region which becomes the flame holder. The correct range of fuel-to-air flow ratio must be internally generated within the recirculation zone to achieve flame stabilization. Fuel is gasified and mixed inside the combustion chamber instead of being premixed with air. Additionally, the fuel-air mixture in the flame holding region is not necessarily at the stoichiometric composition.
The method of fuel addition is in turn dependent upon the combustion process. A turbulent flame transfers heat to the fuel surface (which constitutes the wall of the combustion chamber), causing fuel gasification. The turbulent flame is established between the fuel surface and the boundary layer.
Flame holding in SFRJs occurs in the recirculation zone behind a rearward facing step or sudden expansion region at the heat end of the fuel grain. Heat transfers to the fuel surface during the ignition phase and results in a gasified fuel being injected into the recirculation zone. Simultaneously, a small fraction of the air streaming through the injector becomes entrained and a stable flame is formed in the recirculation zone.
A current hypothesis regarding the flame holding mechanism is that stabilization is achieved when the local mixture ratio in the recirculation zone is within the lean and rich combustion limits of the fuel. For hydrocarbons, this range of stability generally lies between equivalence ratios (.PHI.) of 0.5 and 1.5.
The limiting injection parameter is the dimensionless parameter h/D.sub.p or the A.sub.p /A.sub.i, where h=1/2(D.sub.p -D.sub.i), D.sub.p = diameter of fuel port, D.sub.i = diameter of injector port, A.sub.p = area of fuel port, and A.sub.i = area of injection port. Many fuels of interest have critical (h/D.sub.p) above that which would be desired. This increases the injector pressure drop for fixed D.sub.p and air flow conditions since .DELTA.P.about.D.sub.i.sup.4, where .DELTA.P= injector pressure drop.
Because of the continuing interest in reducing the critical (h/D.sub.p) of ramjet solid fuels, an attempt was made to get a lower value by indirect means. However, the fuel systems having lower critical A.sub.p /A.sub.i have up to 6% lower theoretical performances due to their use of ammonium perchlorate (AP) and HYCAT (a ferrocene compound, burning rate accelerator described in U.S. Pat. No. 3,864,178).
Premature shutdown of a SFRJ is only possible by shutting the air flow off. There are more applications, such as flares or nonpropulsive devices, whose on-off operation and multiple firings are of interest.